1. Field of the Invention
The present invention relates to a light-emitting diode (LED) device and related method for preventing soft-start flicker, and more particularly, to an LED device and related method using a progressively increased load current to perform dimming during a soft-start operation.
2. Description of the Prior Art
Light emitting diodes (LEDs) used as light sources have become popular in recent years. For example, the light source in a back light module of a conventional liquid crystal display (LCD) panel is usually a plurality of cold cathode fluorescent lamps (CCFLs). However, as the luminous efficiency increases and the cost decreases, LEDs have gradually replaced CCFLs to be the light source in a back light module.
In the prior art, an LED driving circuit usually uses pulse width modulation (PWM) mechanism to adjust the brightness of the LED. The PWM dimming primarily takes advantage of a PWM signal to control an average current provided by a current source for the LEDs. When the PWM signal is logical high, the current source is conducted to provide a current for the LED. On the contrary, when the PWM signal is logical low, the current source is stopped providing the current for the LED. Therefore, if the PWM signal stays at logical high longer, the LED gets brighter. In other words, the brightness of the LED can be controlled by changing a duty cycle of the PWM signal.
Please refer to FIG. 1, which is a schematic diagram of an LED driving circuit 10 according to the prior art. The LED driving circuit 10 is utilized for driving an LED module 11. As shown in FIG. 1, the LED module 11 includes parallel-connected LED strings C1˜Cm. Each of the LED strings is composed of multiple LEDs connected in series. The LED driving circuit 10 includes a voltage converter 12, a current load 13, a dimming switch 15 and a loop control unit 14. The voltage converter 12 is utilized for converting an input voltage V1 into an output voltage V2 to drive the LED module 11. The current load 13 is utilized for sinking constant currents Id1˜Idm from the LED module 11. The dimming switch 15 is utilized for conducting or closing couplings between the current load 13 and each of the LED strings C1˜Cm to control the average current of the LEDs. The loop control unit 14 is utilized for controlling the voltage converter 12 to convert voltages according to a voltage difference between the feedback voltage of the LED strings C1˜Cm and a predetermined reference voltage to regulate the voltage level of the output voltage V2. Besides, the loop control unit 14 further includes a dimming control unit 142 for generating a PWM signal SD to implement the dimming procedure.
On the other hand, since the feedback voltages of the LED strings C1˜Cm increase from zero progressively when the system power is on, it is quite different than the predetermined reference voltage of the loop control unit 14. This situation allows the output voltage V2 to go rapidly from logical low to the desired voltage level but the voltage converter 12 may generate a large surge and overshoot during power-on. In this situation, the loop control unit 14 usually includes a soft-start unit 144 to reduce the voltage difference between the predetermined reference voltage and the feedback voltages during power-on, so as to enhance the system stability at start-up time.
However, when the soft-start and PWM dimming both are ongoing, since the current path between the LED module 11 and the current load 13 is conducted and closed alternately but the voltage converter 12 converts voltage only when the current path is conducted, this causes a long soft-start time. In the prior art, the soft-start and PWM dimming are performed separately in different stages. Namely, no PWM dimming is performed during the soft-start operation (the PWM dimming has 100% duty cycle). It is not until the soft-start is completed that the PWM dimming is performed. Such that the long soft-start time can be avoided. However, from no dimming to dimming (e.g. the duty cycle is from 100% to 50%), there may exist a current spike in the average current of the LED, causing soft-start flicker. The more duty cycle has changed from no dimming to dimming, the more the average current of the LED varies. As a result, the soft-start flicker gets more severe at the transient.